Thinking Like a Scientist: Summary
Have you ever wondered
how marine life
fares following an oil spill? How many die?
How many survive to die later? How long does
it take for an ecosystem to
recover? What about the site of the Exxon
Valdez oil spill—has the marine life along the shore recovered after
15 years? As a NOAA scientist, Dr. Alan Mearns has been asking
and answering these types of questions since the spill occurred in March of 1989.
Dr. Mearns is a marine ecologist at NOAA's Hazardous Materials Response Division in the Office of Response and Restoration, part of the National Ocean Service. His office is in Seattle, Washington. His job is to provide scientific information during the cleanup of oil and hazardous substance spills in coastal and marine waters.
His investigations in Prince William Sound
began with two questions:
“Would cleanup methods, such as high-pressure, hot-water washing, speed up the recovery of the shoreline marine life, or would they actually delay the recovery?”
“How did the oil affect the populations of marine life and wildlife of Prince William Sound?”
He didn’t know then that he and his team would spend the next 10 years trying to answer these two basic questions.
Dr. Mearns began by breaking down these two questions into numerous smaller, more answerable questions, which is typical of how most scientific investigations begin. Then, to try to answer these questions, Dr. Mearns and his team used the same approach that most scientists use. First, they read all the relevant published literature about the effects of both oil spills and shoreline cleanup activities on marine life. They found that not much was published on these topics. The information in the literature was not good enough to predict whether the hot-water cleanup method would be effective (in removing oil) or if it would really speed up “recovery.” To find out for sure, they decided that they needed a long-term field-monitoring study.
The study they designed called for monitoring approximately 24 sites in three categories:
Oiled (but not cleaned)
Oiled and cleaned (with high-pressure, hot-water washing)
Unoiled and not cleaned (called a “control”)
This grand experiment had the attributes required of good science: exposed and control sites (for both oiled and cleaned); replication (more than one site in each category); and repeated sampling over a long period. In the spring of 1990, the research team made its first "reconnaissance" trip to Prince William Sound to confirm that the sites were adequate and simply to “learn the territory.” With two boats and a crew of nearly a dozen scientists and technicians, they went out every summer from 1990 to 2000, surveying these sites during the "spring tide" period (a time of very low, low-tides).
To survey marine life on these shorelines,
the scientists used methods that gave them
quantitative data like the kinds of species
present, and their numbers or percent
cover in each sampling area. As the tide flowed out, they randomly selected five or 10 points along a 100-foot line parallel to the water line (called a transect or survey line). They did this at three elevations: upper, middle and low tide elevations. At each point on a rocky shore transect, they laid down a one-quarter-square-meter quadrat (0.25 m on a side). You can see one of these quadrats on the ground in the photo to the right; the transect line is vaguely visible near the left center portion of the photo. Then they identified and counted every plant and animal inside the boundary of the quadrat. In areas of "soft" sediment (sand, gravel, or mud) they took a core sample and then, back at the lab, they carefully sorted through the sample and counted and identified all the clams, amphipods, polychaetes, and other marine organisms.
They found that during the first months after the spill, the oil had killed about half of the marine life on the oiled shores. Washing the shore with high-pressure hot water killed most of what survived the initial oiling. This washing removed a lot of oil, but not all of it. By the second summer (1990), seaweeds, barnacles, snails, limpets and other organisms were coming back, but a lot of oil remained in gravel under the shoreline surface. By the third and fourth summers (1991 and 1992) there was, with notable exceptions, a prolific growth of seaweeds, including rockweed, and intertidal animals at all of the shoreline sites. The cleaned sites actually took a year longer to recover than the oiled but uncleaned sites.
To make sure of their results, they continued the study for another six years. When they reviewed the newer data with the old data, they discovered that there were actually no real differences in the recovery times of the oiled and cleaned sites versus the oiled (but not cleaned) sites. Thus, they had to modify their conclusions and recommendations: heavy cleaning did kill off marine life that otherwise survived the oiling, but the recovery time was about the same. Heavy cleanup made little difference in the end. It certainly did not remove all the oil, nor did it speed up recovery time of the shoreline marine life.
After 10 years, the team was ready to end
its long-term study. However, Dr. Mearns noticed
from the data, and from 10 years of shoreline
photos, that something else was going on with the intertidal marine
life in Prince William Sound. It looked like mussels,
seaweeds, clams, barnacles and other organisms
were going through a series of cycles. In some
years, the shores were covered with mussels.
Yet, in other years, mussels were nearly absent
and the sites were thick with seaweed. The photos from one site,
which featured a large boulder, clearly showed this long-term variation. Another NOAA biologist gave this boulder a name: Mearns Rock! Since 2000, Dr. Mearns and several colleagues have continued to return and re-photograph a dozen oiled sites in Prince William Sound, including his namesake rock.